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<br />30 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />maximum seeding effect is expected near or between these two gauge lines. Additional gauges <br />will be located at other downwind distances, along the prevailing wind direction, to help <br />estimate the area affected by seeding. Gauges will be able to resolve to within 5 min and 0.1 <br />mm water equivalent. All gauges will be located in sheltered locations and will be equipped <br />with shields to minimize losses caused by wind effects. <br /> <br />A very important aspect of the direct detection experiments will be the estimation of which <br />gauges were under the seeding plume(s) and which were clearly crosswind. Aircraft monitoring <br />can document the seeding plumes near their tops, about 600 m above the surface targets. <br />However, seeding-caused snowflakes will not settle to the surface directly under the plume at <br />aircraft levels because of directional wind shear related to surface friction. For example, with <br />westerly winds aloft, near-surface winds would be expected to be approximately from the <br />southwest. <br /> <br />Doppler radar monitoring of ice crystals and chaff will provide wind information and estimated <br />plume position in the important layer between the aircraft and target surface. The radar <br />profiler and rawinsonde observations also will help in assessment of plume position with time. <br />This assessment will indicate which gauges were possibly affected by seeding and which were <br />not. Gauges usually or always under the near-surface meandering plume for the duration of an <br />experiment will be considered seeded gauges. Those gauges sometimes under the plume will be <br />considered possibly affected and will be considered to be in a "buffer zone". Gauges definitely <br />crosswind of the plume throughout an experiment will be used as nonseeded control gauges. <br /> <br />The transport of seeding agents and resulting precipitation to the surface can be inferred by <br />chemical examination of the precipitation. For example, the silver concentration in <br />precipitation can indicate whether targeting succeeded or not, even though much or all of the <br />silver may have been scavenged by natural precipitation. A better approach, discussed by <br />Warburton et al. (1989), is to examine the ratio of co-released silver (from AgI) and indium <br />(from indium oxide). The latter does not nucleate ice, but can be released in size distributions <br />similar to AgI by remote-controlled generators like AgI seeding generators. If silver and indium <br />are released at the same rate, a silver-to-indium ratio well above 1 indicates AgI was not only <br />scavenged (presumably at the same rate as the indium), but was involved in ice crystal <br />nucleation with resulting hydrometeors falling on the sampling sites. <br /> <br />Radio-controlled snow sampling devices will be developed and co-located with each <br />precipitation gauge. A single snow sampler will be exposed throughout each AgI direct <br />detection experiment. They will be serviced soon after each experiment or series of experiments <br />to prevent melting of the snow. Each snow sample will later be analyzed for silver and indium <br />concentration. Samples with only background levels of both elements will be assumed to be <br />natural snowfall. <br /> <br />Only indium will be released during propane seeding experiments. With either type of seeding, <br />chemical analysis of snow samples will provide another means of identifying which gauges were <br />clearly under the seeding plumes and which were not. With AgI seeding, high silver-to-indium <br />ratios will indicate that AgI nucleation created some of the snowfall (mass unknown). Ratios <br />near one will suggest that only scavenging brought the chemicals to the surface. With propane <br />seeding, the presence of indium will indicate the site was under the seeding plume. This <br />chemical approach will serve as an excellent crosscheck on the physical measurement (winds, <br />plume tracking) method of estimating which gauges were seeded and which were not. Silver <br />